and on the Function of the Antenna in Crustacea. 43 



sure derivable from sounds. To animals so low in the scale of 

 beings as the Crustacea, placed as they are in a medium which 

 must considerably modify its character, sound can convey little 

 to the consciousness of the animal beyond a sense of security or 

 danger. 



To enable this to be of the most extensive value, the auditory 

 organ must and always is so placed as to be most exposed to 

 external impressions at all periods ; particularly when the animal 

 is at rest or pre-occupied. 



Now if we look at the organ the present state of science attri- 

 butes to the sense of hearing, we find that in the most perfectly 

 formed animals, the Bi'achyura, it is enclosed within a bony case 

 and secured by a calcareous operculum ; that it is always so in a 

 state of rest, and only exposed when especially required. Not 

 only is this the case throughout the order, but in some genera, 

 as Corystes, Cancer, &c., it is again covered by the supplying 

 organs of the mouth. 



If we take into consideration that the inferior antenna is fre- 

 quently developed into organs assimilating to feet, and fre- 

 quently used for the purpose of assisting to climb, &c., it seems 

 difficult to admit that it is an organ capable of protecting the 

 animal by its quick detection of the sound of approaching 

 danger. 



If we turn our attention to the superior antenna, we find that 

 in the living animal it is always elevated in the water and never 

 at rest, — always playing with a constant vibration and a jerking 

 motion peculiar to itself in the higher orders. Among the Am- 

 phipoda, though constantly erect, the motion is more regular and 

 graceful ; this probably is consequent upon the greater relative 

 length of the organ. 



This organ is one that appears as if always on the watch ; — let 

 the animal be at rest, let it be feeding, no matter, the superior 

 antenna is ever elevated and on constant guard. 



Again, if we turn our attention to the land Crustacea, we find 

 the organ as an antenna disappear; and in Ligia and the amphi- 

 bious Orchestida they are rudimentaiy, as if the organ, passing 

 from water into a less dense medium, required modification in 

 order to adapt it to the change of circumstances. 



If we take into consideration the nature of sound, and its dif- 

 ference of character when conveyed under water from that of 

 passing through air, the obtuse character of the former, which 

 can scarcely be more than a vibratory action of particles of 

 water, which conveys to us a very modified and imperfect idea 

 of sound, we find it difficult to understand that the organ 

 situated at the base of the under antenna is capable of receiving 



